Mini-tension tester

ABSTRACT

A compact tension testing apparatus for determining the peel strength of a circuit line or film bonded to a substrate. A force gauge is mounted on an apparatus using low friction air bearings to allow the force gauge to self-align with the peeling location of the circuit line or film, thereby providing essentially orthogonal positioning relative to the substrate.

This application is a continuation of U.S. patent application Ser. No.09/211,975, filed on Dec. 15, 1998, now U.S. Pat. No. 6,185,999.

FIELD OF THE INVENTION

The present invention is in the field of tensile testing machines. Moreparticularly, the present invention provides an improved apparatus andmethod for measuring the peel strength of a circuit line or film bondedto a substrate such as a circuit board. Also, the present inventionrelates to universal testing machines providing tensile, compressive,shear, bending, and torsion tests on a material sample.

BACKGROUND OF THE INVENTION

Tensile testing machines are commonplace. Typically, a material testsample is clamped to a horizontal platform, and a clamping deviceattached to a force gauge is lowered in a vertical direction andattached to the test sample. Means are provided to move the clampingdevice and force gauge in an upward direction, thereby causing a tensileforce to be applied to the test sample.

Commonly, samples are destructively tested in a large tensile testingmachine, wherein a sample to be tested must be sacrificially extractedfrom a larger specimen for the sake of the test. Once the test has beencompleted, the sample is usually discarded, which can be prove to bevery costly.

Typical tensile testing machines used for laminate bond testing onlyprovide peeling movement in one direction, so that the electricalcircuit line or other sample on a substrate being tested must be linedup along the one direction of movement. Unfortunately, this requires thesubstrate to be repositioned and clamped whenever a circuit line has adirectional orientation different from the previous test direction.Further, when peeling a circuit line from a substrate, the force gaugemust be constantly moved in order to keep the force gauge directly overthe peeling location. This is necessary in order to ensure that a trueforce reading of the force perpendicular to the substrate is beingmeasured.

SUMMARY OF THE INVENTION

The present invention avoids the disadvantages of the prior art byproviding a compact mini-tension tester. The mini-tension testerincludes a base plate, a x-axis slide apparatus, a y-axis slideapparatus, a z-axis slide apparatus, a servo actuator assembly, a forcegauge, a cable, and a gripper clamp.

A substrate with a film or circuit line bonded to its surface can beattached to the base plate using clamps, vacuum means, or otherattaching systems. The z-axis slide apparatus is slidably attached tothe y-axis slide apparatus and the y-axis slide apparatus is slidableattached to the x-axis slide apparatus. Air bearings, or otherfrictionless type mechanisms, are used to provide essentially frictionfree motion. Therefore, the z-axis slide apparatus can move essentiallyfriction free to any location within the x and y plane.

A servo actuator assembly is attached to the z-axis slide apparatus, anda force gauge is attached to the servo actuator assembly. The forcegauge preferably comprises a strain gauge load cell, although othertypes of force measurement devices may be used. A cable connects theforce gauge to a gripper clamp that is attached to a test sample locatedon a substrate. The test sample may include, for example, a circuit lineor film formed on a substrate such as a printed circuit board.

In order to measure the pull force required to pull a circuit line orfilm from a substrate, the substrate is firmly attached to the platformusing a vacuum system. In order to obtain test data without destroyingthe substrate or affecting the operation of the circuitry on thesubstrate, sample test circuit lines are applied to the substrate duringthe production process. Preferably, the sample test circuit lines areonly used to monitor the production process, and are not involved withany part of the electronic functioning of circuitry on the substrate.Therefore, these circuit lines may be peeled off the substrate fortesting, without sacrificing the operational circuitry on the substrate.

During testing, the end of a circuit line is peeled from the substrateand grasped by the gripper clamp. Next, a servo actuator assembly in thez-axis slide apparatus displaces the force gauge, cable, and gripperclamp upward in the z-direction at a constant velocity, therebyproviding an upward force that peels the circuit line away from thesubstrate. The servo actuator assembly is force limited to provide amaximum of about 20 pounds of force. The desired force reading is theforce applied in a direction perpendicular to the substrate. In thepresent invention, the friction free air bearings in the x-axis slideapparatus and y-axis slide apparatus allow the z-axis slide apparatus to“walk” with the circuit line release or peel point. This ensures thatthe force applied to the release point of the circuit line is alwaysperpendicular to the substrate. Therefore, the force gauge is alwaysmeasuring the desired force, that is, the force perpendicular to thesubstrate.

The use of the mini-tension tester is not restricted to only providingtensile testing, but can also provide compressive, shear and bendingmaterial testing, and strength testing. For instance, compressivetesting can be conducted by providing a rigid member between the forcegauge and the test object. Then the force cell is moved along the z-axisdirection toward the test object, thereby creating a compressive forceon the test object. For applying shear force, a test object can beclamped onto the base plate in a direction such that the desired shearforce is in line with the z-axis of the mini-tension tester. In anotherembodiment, a shear force can be applied to the test object by attachingthe servo actuator assembly in a direction perpendicular to the z-axis.For this case, a shear force can be applied to a test object in adirection parallel to the base plate. If a test object is attached tothe base plate in a cantilevered manner, the servo actuator assembly canapply a force in the z-axis direction to the free end of the cantilevercausing a bending moment in the test object.

The present invention additionally provides a mini-tension tester thatis compact enough to fit inside an oven to provide elevated temperaturetesting. The mini-tension tester is portable and versatile since avariety of substrate sizes can be attached to the base plate. Also, themini-tension tester is much less costly then the large tensile testingmachines that it replaces.

Generally, the present invention provides an apparatus for measuring thepeel strength of a material bonded to a substrate, comprising:

a gripper clamp for grasping a material bonded to a surface of asubstrate;

a force gauge attached to a z-axis displacement system and coupled tothe gripper clamp, wherein a displacement of the z-axis displacementsystem causes the material to peel away from the substrate; and

x and y-axis displacement systems attached to the z-axis displacementsystem for providing self-aligning orthogonal positioning of the forcegauge relative to a release point of the material as the material ispeeled away from the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention will best be understood from adetailed description of the invention and a preferred embodiment thereofselected for the purposes of illustration and shown in the accompanyingdrawings in which:

FIG. 1 illustrates a mini-tension tester according to a preferredembodiment of the present invention;

FIG. 2 illustrates a top perspective view of the mini-tension tester ofFIG. 1; and

FIG. 3 illustrates a graph of force versus peel distance along a circuitline.

DETAILED DESCRIPTION OF THE INVENTION

The features and advantages of the present invention are illustrated indetail in the accompanying drawings, wherein like reference numeralsrefer to like elements throughout the drawings.

A mini-tension tester 10 for measuring the peel strength of a film 12bonded to a substrate 14 in accordance with a preferred embodiment ofthe present invention is illustrated in detail in FIGS. 1 and 2. Themini-tension tester 10 generally includes a base plate 16, a vacuumsurface 18, an x-axis slide apparatus 20, a y-axis slide apparatus 22, az-axis slide apparatus 24, a servo actuator assembly 26, a servoactuator controller 28, a force gauge 30, a cable 32, and a gripperclamp 34.

The x-axis slide apparatus 20 includes air bearing slides 32 and 31, airbearings 36 and 38, and bridge 40. Air bearings 36 and 38 are attachedto the bridge 40 and slide essentially “friction free” on top of the airbearing slides 32 and 31. Therefore, the x-axis slide apparatus 20allows essentially “friction free” motion of the bridge 40, in the “x”direction 42 as shown in FIG. 2.

The y-axis slide apparatus 22 includes an air bearing slide 44 and anair bearing 46. The air bearing 46 slides essentially “friction free” ontop of the air bearing slide 44 in the “y” direction 48 as shown in FIG.2.

The z-axis slide apparatus 24 includes a vertical post 50, a slide 52,and the air bearing 46. Air bearing 46 is rigidly attached to thevertical post 50. Slide 52 is slidingly attached to the vertical post50. The force gauge 30 is attached to the slide 52. Servo actuatorassembly 26 provides controlled relative motion between the slide 52 andthe vertical post 50. Cable 32 connects the force gauge 30 to thegripper clamp 34 (FIG. 1).

The servo actuator controller 28 provides control signals to the servoactuator assembly 26 through control cable 54 to control thedisplacement of the slide 52 and attached force gauge 30 relative to thesubstrate 14. Preferably, a constant velocity motion is generatedbetween the substrate 14 and the force gauge 30. However, a variablevelocity motion may be used, depending on the type of testing beingperformed by the tester 10. As shown in FIG. 1, for example, with thegripper clamp 34 grasping the end 33 of the film 12 on the substrate 14,a constant velocity motion provided by the servo actuator assembly 26results in the film 12 being peeled from the substrate 14 at a constantvelocity. As the film 12 is being peeled from the substrate 14, theoutput from the force gauge 30 provides a continuous measurement of theforce being applied to the film 12. Advantageously, the x-axis slideapparatus 20 and y-axis slide apparatus 22 are configured tocontinuously position, i.e., self-align, the force gauge 30 directlyabove the release point of the film 12 on the substrate such that theforce gauge 30 is always measuring a force perpendicular to thesubstrate.

Referring again to FIG. 1, a computer or other type of processing system76 can be used to gather force measurement data through cable 70 andpositional data through cable 72. Cable 70 connects the force gauge 30to the computer 76, and cable 72 connects the servo actuator controller28 to the computer 76. The servo actuator controller 28 providesinformation regarding the position and movement of the servo actuatorassembly 26. Therefore, the computer 76 can be used to gather the forcemeasurement, along with time and displacement measurements, as a test isbeing conducted.

FIG. 2 illustrates the peel strength testing of a circuit line 60 on asubstrate 14. The circuit line 60 may be a sample test line or maycomprise a portion of the operational circuitry on the substrate 14.Initially, a first end 62 of the circuit line 60 is peeled off of thesubstrate 14 and gripped by the gripper clamp 34. Next, the servoactuator assembly 26 in the z-axis slide apparatus 24 displaces theslide 52, force gauge 30, cable 32 and gripper clamp 34 at a constantvelocity in an upward “z” direction 64 (see FIG. 1). This upward motionprovides an upward force that peels the circuit line 60 away from thesubstrate 14 (FIG. 2). Since the force gauge 30 is positioned above therelease point of the circuit line 60, the force measured by the forcegauge 30 is the force applied to the circuit line 60 in the “z”direction 64 that is perpendicular to the substrate 14.

As the slide 52, force gauge 30, cable 32 and gripper clamp 34 continueto be displaced at a constant velocity in an upward “z” direction, thez-axis slide apparatus 24 “walks” with the circuit line 60 releasepoint, even if the circuit line 60 changes direction (FIG. 2). That is,the force required to peel the circuit line 60 away from the substrate14 additionally causes the z-axis slide apparatus 24 to be pulled alongwith, and continuously positioned above, the release point of thecircuit line 60. Such self-aligning displacement of the z-axis slideapparatus 24 is provided through the use of the air bearing structure ofthe x-axis slide apparatus 20 and the y-axis slide apparatus 22.Therefore, essentially “friction free” motion of the z-axis slideapparatus 24 is provided in the “x-y” plane. Thus, in the preferredembodiment of the present invention, the force gauge 30 is alwaysmeasuring the force that is perpendicular to the substrate 14.

Referring to FIG. 3, a graph of the force measured by the force gauge 30versus the peel distance along the substrate 14 can be used forevaluation of the bonding strength along the circuit line 60. If thebonding strength is uniform along the circuit line 60, the graph offorce versus peel distance will form an essentially horizontal line asillustrated in region (A) on FIG. 3. If the bonding strength is higherin one region along the circuit line 60, then the graph of force versuspeel distance will form an upward spike, as illustrated in region (B) onFIG. 3. If the bonding strength is lower in one region along the circuitline 60, then the graph of force versus peel distance will form adownward spike, as illustrated in region (C) on FIG. 3. Therefore, thegraph of force versus peel distance provides information on the qualityof the bonding strength along the circuit line 60.

The cable 32 preferably has a predetermined minimum length to limit theeffect of a temporary deflection of the cable 32 on the force valuemeasured by the force gauge 30. Such a temporary deflection may occur,for example, if a large section of the circuit line 60 suddenly releasesfrom the substrate 14 during testing. The minimum length of the cable 32is chosen to minimize the deviation of the force application angle onthe force gauge 30. In the preferred embodiment of the presentinvention, a minimum cable length of about 18 inches has proven to beadequate.

The foregoing description of the present invention has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andmany modifications and variations are possible in light of the aboveteaching. Such modifications and variations that may be apparent to aperson skilled in the art are intended to be included within the scopeof this invention as defined by the accompanying claims.

We claim:
 1. An apparatus for measuring the peel strength of a material bonded to a substrate, comprising: a gripper clamp for grasping a material bonded to a surface of a substrate positioned in an x-y plane; a force gauge attached to a z-axis displacement system and coupled to the gripper clamp, wherein a displacement of the z-axis displacement system causes the material to peel away from the substrate; and x and y-axis displacement systems attached to the z-axis displacement system for providing self-aligning orthogonal positioning of the force gauge relative to a release point of the material as the material is peeled away from the substrate.
 2. The apparatus according to claim 1, wherein the material is a circuit line.
 3. The apparatus according to claim 1, wherein the material is a film.
 4. The apparatus according to claim 1, wherein the substrate is a circuit board.
 5. The apparatus according to claim 1, wherein the x and y-axis displacement systems are configured to provide substantially frictionless displacement of the z-axis displacement system relative to the substrate.
 6. The apparatus according to claim 1, wherein the z-axis displacement system comprises: a movable member for supporting the force gauge; and a system for displacing the movable member along the z-axis.
 7. The apparatus according to claim 6, wherein the x-axis displacement system comprises: a plurality of air bearing slides; a bridge; and a plurality of air bearings for supporting the bridge on the plurality of air bearing slides.
 8. The apparatus according to claim 7, wherein the y-axis displacement system comprises: an air bearing slide mounted to the bridge; and an air bearing attached to the movable member of the z-axis displacement system and coupled to the air bearing slide mounted to the bridge.
 9. The apparatus according to claim 1, further including a cable for coupling the force gauge to the gripper clamp.
 10. The apparatus according to claim 1, further including a control system for controlling operation of the z-axis displacement system.
 11. The apparatus according to claim 1, further including a processing system for recording force information provided by the force gauge and data corresponding to displacement of the z-axis displacement system in the x-y plane.
 12. A method for measuring the peel strength of a material bonded to a substrate positioned in a plane, comprising the steps of: providing a force gauge and coupling the force gauge to the material; displacing the force gauge orthogonally relative to the plane of the substrate to peel the material away from the substrate and to measure the force required to peel the material away from the substrate; and orthogonally self-aligning the force gauge relative to the plane of the substrate as the material is peeled away from the substrate to maintain the force gage above a release point of the material.
 13. The method according to claim 12, further comprising the steps of: mounting the force gauge on a displacement system; and providing substantially frictionless displacement of the displacement system relative to the substrate to provide the orthogonal self alignment of the force gauge.
 14. An apparatus for measuring physical properties of materials comprising: a system for applying a first force to an object positioned in a plane, wherein the force is applied in a direction substantially orthogonal to the plane; a system for measuring a physical property of the object in response to the applied first force; and a system for self-aligning in at least three degrees of freedom and applying a second force to the object.
 15. An apparatus for measuring a characteristic of a material comprising: a system for applying an orthogonal first force to a testing point of a material; a system for measuring a characteristic of the material in response to the applied first force; and a system for self-aligning in at least three degrees of freedom and applying a second force to the testing point of the material.
 16. An apparatus for measuring physical properties of materials comprising: a system for applying a first force to an object positioned in a plane, wherein the first force is applied in a direction substantially orthogonal to the plane; a system for measuring a physical property of the object in response to the applied first force; and a system for frictionally self-aligning in at least three degrees of freedom and applying a second force above the object in response to a displacement of the object within the plane.
 17. A method for measuring a characteristic of a material, comprising the steps of: applying an orthogonal force to a testing point of the material; measuring a characteristic of the material in response to the applied force; and self-aligning the force applying system above the testing point of the material in response to a displacement of the testing point.
 18. A method for measuring physical properties of materials comprising the steps of: applying a first force to an object positioned in a plane in a direction substantially orthogonal to the plane; measuring a physical property of the object in response to the applied first force; and self-aligning in at least three degrees of freedom and applying a second force to the object. 